(1) Field of the Invention
The present invention relates to a digital receiver that receives a digital-modulated transmitted wave.
(2) Description of the Related Art
In recent years, the digitization of CATV broadcasts is being promoted. Video and audio signals are usually encoded by a digital encoding method, such as MPEG (Motion Picture Experts Group), to generate a digital stream, which is transmitted by a digital modulation method. In addition, with the object of bringing a pay broadcast into operation, a digital stream is scrambled on the transmitting station side. More specifically, the digital stream is encrypted. By performing descramble processing to decrypt the digital stream on the receiver side, only persons who hold a contract can view the broadcast. This is how a system is configured. This scrambling/descrambling method is called CA (Conditional Access). Because CATV broadcasting companies can individually adopt an arbitrary CA method, a broadcast transmitted by each broadcasting company can be received only by receivers including CA corresponding to the broadcasting company in question. Therefore, if the CATV broadcasting company is changed, the receiver therefor also needs to be changed to a receiver that includes CA corresponding to the new broadcasting company. Taking such situations into consideration, what is being developed is a technology in which instead of including CA in a receiver, the CA is configured as an external processing module that can be replaced. For example, in the standards disclosed in Open Cable (TM) Cable CARD (TM) Interface Specification OC-SP-CC-IF-116-040402, Cable Television Laboratories, Inc. 2004, replacing a processing module makes it possible to view broadcasts transmitted by each CATV broadcasting company. In the standards, CA is included in a processing module called POD (Point Of Deployment). Here, by replacing POD with that corresponding to each CATV broadcasting company, it becomes possible to view broadcasts transmitted by the CATV broadcasting company in question without changing a receiver itself.
In the above-mentioned standards that use POD, a channel which transmits from a transmitting station a digital stream to be actually viewed is called FAT (Forward Applications Transport); and QAM (Quadrature Amplitude Modulation) is used as a modulation method. In addition to the FAT channel, there is a subchannel called OOB (Out Of Band), a frequency of which differs from that of the FAT channel. This subchannel is used to transmit additional information. The OOB comprises a downstream channel called FDC (Forward Data Channel), and an upstream channel called RDC (Return Data Channel). What is transmitted from a transmitting station through the FDC includes a channel map relating to a FAT channel, and additional information such as encryption information. What is transmitted from a receiver to the transmitting station through the RDC includes information about the receiver. However, it is not always necessary to use the RDC. QPSK (Quadrature Phase Shift Keying) is used as a modulation method of FDC. A QPSK receiving circuit of FDC is required independently of a QAM receiving circuit for receiving FAT.
In the meantime, as an example of using a decryption module (POD), for example, Japanese Patent Laid-Open No. 2001-160961 discloses a configuration in which a judgment is made as to whether or not video data is scrambled, and if it is judged that the video data is not scrambled, the power supply to a descrambling unit is stopped. In other words, if descrambling is not required, the power supply to the descrambling unit is stopped so that the power consumption is reduced.